WASHINGTON—It's not every day that Washingtonians have to fight for an open chair at the Brookings Institution for a Wednesday-afternoon foreign policy event. But since it was Florida Sen. Marco Rubio doing the talking, two days after a campaign trail appearance with Mitt Romney, the think tank had a barnstormer on its hands.

Rubio, a young, charismatic Republican at the center of speculations over Romney's running mate, arrived onstage with Connecticut Sen. Joseph Lieberman, a former Democratic vice presidential candidate, to show off his foreign policy chops.

Because of this guessing game, almost every word Rubio utters in public from now until the moment Romney makes his choice will be dissected, prodded and scrutinized. Whether he likes it or not, events like these are the start of a long spring training.

Every seat was filled, with reporters and onlookers lining the walls. An overflow space with the speech displayed on a screen was set up down the hallway. Thousands more tuned in on computer live streams. Due to a reported threat to Rubio this week, security agents stood on watch outside the building.

In his speech, Rubio outlined an aggressive vision of American foreign policy, criticizing his colleagues on the left and the right for supporting a restrained future for the nation. He took great care not to draw many obvious and sharp contrasts with President Barack Obama, and pointed out where he disagreed with members of his own party on American action abroad.

"I always start by reminding people that what happens all over the world is our business," Rubio said of those who ask him why America doesn't restrict itself to domestic affairs.

Rubio went on to advocate for stopping Iran from obtaining nuclear weapons—a view he shares with the president—playing a stronger role in advocating free trade in the Western Hemisphere (except for his parents' native Cuba, where he supports the ongoing embargo), and intervening globally to further American interests, even if it means going it alone.

"America has acted unilaterally in the past—and I believe it should continue to do so in the future—when necessity requires," he said. "But our preferred option since the U.S. became a global leader has been to work with others to achieve our goals."

During a time in which American politicians restrict their attention largely to domestic issues in the aftermath of the last recession, Rubio stands out as a lawmaker openly willing to pour taxpayer resources into foreign projects and initiatives. He called foreign aid—which many Republican presidential candidates said should be cut during presidential debates last year—"a very cost-effective way" to advance American interests and touted the advancement of taxpayer-backed humanitarian missions.

"Faced with historic deficits and a dangerous national debt, there has been increasing talk of reducing our foreign aid budget. But we need to remember that these international coalitions we have the opportunity to lead are not just economic or military ones," he said. "They can also be humanitarian ones as well. In every region of the world, we should always search for ways to use U.S. aid and humanitarian assistance to strengthen our influence, the effectiveness of our leadership, and the service of our interests and ideals."

"I disagree with voices in my own party who argue we should not engage at all, who warn we should heed the words of John Quincy Adams not to go 'abroad, in search of monsters to destroy,'" he said.

Near the end of his address, Rubio, who was reading his speech from papers on the lectern, paused and looked around.

"I left the last page of my speech," he said, flipping through the papers and looking to Lieberman for some help. "Does anyone have my last page?"

A man on the stage handed him the page, and Rubio went on to finish his address, quoting former British Prime Minister Tony Blair about how interventionism is part of America's "destiny."

"This is why I need this page," he said smiling, adding later: "I couldn't memorize a Tony Blair quote."

After the event, Lieberman offered some advice for the vice presidential contenders before getting into the car that would take him back to Capitol Hill.

"Continue to be who you are," he advised. "Really. And history—fate will work its will."